The flatworm mitochondrial genome: an expansion into the phylogeny and molecular epidemiology of taeniid tapeworms

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The mitochondrial genetic code of cestodes was presumed. In the Platyhelminthes the codon UAA was assigned as tyrosine; however, it became clear that it is the termination codon like other metazoan animals. Moreover, we defined that the codon GUG serves as an initiation codon.The complete DNA sequences of the mitochondrial genomes (mtDNA) of the tapeworm Echinococcus multilocularis and Taenia solium have been determined. The E, multilocularis mtDNA is a circular genome (13,738 bp in length) and contains 36 genes (12 for proteins involved in oxidative phosphorylation, 2 for rRNAs and 22 for tRNAs) but a gene for ATP8 is missing. All genes are transcribed in the same direction. The gene arrangement is characteristic but the partial organization between ND1 and Cox1 genes is similar to those of annelid mtDNAs, suggesting that the Platyhelminthes is derived within the Eutrochozoa. The T. solium mtDNA is 13,709 bp in length and its gene arrangement is identical with that of the E. multilocularis mtDNA. In a gene for ND1, the abbreviated stop codon U was confirmed for the first time in flatworm mtDNAs. A sequence analysis of CDNA demonstrated that the complete termination codon UAA is created post-transcriptionally by polyadenylation.Genetic polymorphism was determined among geographic samples of T. solium, using PCR-amplified sequences of Coxl and Cytb genes. The Asian samples from China, Thailand, Irian Jaya, and India formed a single cluster; however, the Latin American samples from Mexico, Peru, Ecuador, Bolivia, and Brazil combined with the African samples from Tanzania, Mozambique, and Cameroon to form an additional cluster. These results and historical data of swine domestication suggest that T. solium was introduced recently into Latin America and Africa from different parts of Europe and that the tapeworm of another origin was independently spread in Asian countries.